In general, a conventional speed reducer is driven through a construction that includes a pinion shaft configured to receive power of a motor, a speed reduction gear disposed at one end of the pinon shaft and formed integrally therewith, a ring gear configured to mesh with the speed reduction gear so as to be rotated therewith, and a ring gear shaft fixed to the inner surface of the ring gear in a pin-coupling manner and having therein a pin hole for transmitting the received power to a driving shaft.
In this conventional speed reducer, because an output shaft is coupled to the ring gear shaft by means of a pin, the link therebetween is not firm, and thus power transmission is not smoothly realized. Further, the pin, the ring gear shaft and the output shaft do not withstand rotational force well and are frequently damaged upon long-term operation, leading to failure of the speed reducer.
Further, the ring gear shaft and the ring gear, which are typically fixed to each other using a key, do not withstand rotational force well, and are frequently damaged upon long-term operation of the speed reducer, leading to an increase in the cost of maintaining the speed reducer.
Furthermore, because the output shaft is not securely fixed, the related art has a limitation in transferring minute rotational force due to undesirable movement and uneven wear of the output shaft and has a very complicated structure and a large overall volume, whereby it is a hard task to apply the related art to devices that need to be precisely controlled in industrial machines, e.g. joints of robots, and manufacturing costs thereof are high.
A speed reducer disclosed in Korean no. Laid-open Publication No. 2014-0096683 has been developed in view of the above problems, and this speed reducer includes a driving shaft configured to be rotated by a driving unit, a pair of driving gears including a first driving gear and a second driving gear, which are bevel gears mounted on the end of the driving shaft so as to face each other and having different sizes from each other and the same gear ratio, a pair of driven gears including a first driven gear and a second driven gear, which are bevel gears mounted so as to face each other and respectively mesh with the first driving gear and the second driving gear, a driven shaft configured to support the first driven gear and the second driven gear and to allow the driving shaft to penetrate the middle portion thereof, and a rotary member connected to the driven gears and configured to output rotational force with a speed reduced, whereby the speed reducer is reduced in volume and is precisely operated.
However, the aforementioned related art for speed reduction is configured such that the driving gears, which have the same gear ratio and different sizes from each other, are mounted on the driving shaft so as to face each other and such that the driven gears, which have the same gear ratio and different sizes from each other, are mounted on the driven shaft so as to face each other. Further, because the reduction gear ratio is determined depending on the sizes of the driven gears, the volume of the device increases in order to realize a desired speed reduction, and from a structural aspect it is difficult to apply the related art to a rotary member having a large weight.